1. l Field of the Invention
This invention relates to a distance sensor for a servo control system and particularly to a non-contact capacitance probe apparatus and an improved capacitance sensing circuit that, illustratively, may be employed in an optical playback and record system as a distance sensor for maintaining a lens system within a particular depth of focus.
2. Description of the Prior Art
In one illustrative process of measuring the distance between a capacitance probe and a varying object, such as a rotating disc record, the sensing probe that is made of an electrically conductive material is secured to a lens system such that an electrically conductive coating on the disc record and the probe form a capacitance element. This capacitance element is futhermore connected into an electronic circuit that converts the variations in capacitance values of the element into a control signal for controlling a position actuator that is used to position the lens system relative to a surface of the rotating disc. The electronic circuit includes, illustratively, a fixed frequency oscillator and an FM ratio detector wherein the capacitance element is coupled into the detector which has a natural frequency that depends on the value of the capacitance element. When the surface of the disc is at the desired distance from the lens system, the fixed frequency of the oscillator will correspond to the natural frequency of the ratio detector. However, when the fixed frequency of the oscillator does not correspond to the natural frequency of the detector, an error signal is produced that is used to control the position actuator.
In similar circuits using a sensor comprising a single electrode probe, difficulties have been encountered in providing an adequately well grounded disc. Any variation of impedance in the path which grounds the disc are manifested as spurious signals from the sensor.
There is a need for an improved capacitance sensor that is nearly linear over a relatively large range of distances including distances approaching zero.
There is a further need for capacitance sensor circuitry and probe geometry that is insensitive to variations in ground return inductance. More particularly, a sensor is needed that may be operated relatively free of interaction with a second independent sensor of similar design if two sensors are employed simultaneously for playing back or recording information on the same disc.